[PATCH] x86-64: fix combining of regions in init_memory_mapping()

[PATCH] x86-64: fix combining of regions in init_memory_mapping()

[Jan Beulich]

When nr_range gets decremented, the same slot must be considered for
coalescing with its new successor again.

Signed-off-by: Jan Beulich <jbeulich@novell.com>

---
 arch/x86/mm/init_64.c |    2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

--- linux-2.6.27-rc6/arch/x86/mm/init_64.c	2008-08-29 10:53:00.000000000 +0200
+++ 2.6.27-rc6-x86_64-mr-coalesce/arch/x86/mm/init_64.c	2008-09-12 11:58:45.000000000 +0200
@@ -636,7 +636,7 @@ unsigned long __init_refok init_memory_m
 		old_start = mr[i].start;
 		memmove(&mr[i], &mr[i+1],
 			 (nr_range - 1 - i) * sizeof (struct map_range));
-		mr[i].start = old_start;
+		mr[i--].start = old_start;
 		nr_range--;
 	}
 

Re: [PATCH] x86-64: fix combining of regions in init_memory_mapping()

[Yinghai Lu]

On Fri, Sep 12, 2008 at 7:43 AM, Jan Beulich <jbeulich@novell.com> wrote:
> When nr_range gets decremented, the same slot must be considered for
> coalescing with its new successor again.
>
> Signed-off-by: Jan Beulich <jbeulich@novell.com>
>
> ---
>  arch/x86/mm/init_64.c |    2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> --- linux-2.6.27-rc6/arch/x86/mm/init_64.c      2008-08-29 10:53:00.000000000 +0200
> +++ 2.6.27-rc6-x86_64-mr-coalesce/arch/x86/mm/init_64.c 2008-09-12 11:58:45.000000000 +0200
> @@ -636,7 +636,7 @@ unsigned long __init_refok init_memory_m
>                old_start = mr[i].start;
>                memmove(&mr[i], &mr[i+1],
>                         (nr_range - 1 - i) * sizeof (struct map_range));
> -               mr[i].start = old_start;
> +               mr[i--].start = old_start;
>                nr_range--;
>        }
>

this patch seems not right.
Ingo, please don't apply it.

original code:
        /* try to merge same page size and continuous */
        for (i = 0; nr_range > 1 && i < nr_range - 1; i++) {
                unsigned long old_start;
                if (mr[i].end != mr[i+1].start ||
                    mr[i].page_size_mask != mr[i+1].page_size_mask)
                        continue;
                /* move it */
                old_start = mr[i].start;
                memmove(&mr[i], &mr[i+1],
                         (nr_range - 1 - i) * sizeof (struct map_range));
                mr[i].start = old_start;
                nr_range--;
        }

so it save old_start and first, and move entries forward (so old one
is overwriten), and put back old_start ...

YH

Re: [PATCH] x86-64: fix combining of regions in init_memory_mapping()

[Jan Beulich]

>>> "Yinghai Lu" <yhlu.kernel@gmail.com> 14.09.08 20:20 >>>
>On Fri, Sep 12, 2008 at 7:43 AM, Jan Beulich <jbeulich@novell.com> wrote:
>> When nr_range gets decremented, the same slot must be considered for
>> coalescing with its new successor again.
>>
>> Signed-off-by: Jan Beulich <jbeulich@novell.com>
>>
>> ---
>>  arch/x86/mm/init_64.c |    2 +-
>>  1 file changed, 1 insertion(+), 1 deletion(-)
>>
>> --- linux-2.6.27-rc6/arch/x86/mm/init_64.c      2008-08-29 10:53:00.000000000 +0200
>> +++ 2.6.27-rc6-x86_64-mr-coalesce/arch/x86/mm/init_64.c 2008-09-12 11:58:45.000000000 +0200
>> @@ -636,7 +636,7 @@ unsigned long __init_refok init_memory_m
>>                old_start = mr[i].start;
>>                memmove(&mr[i], &mr[i+1],
>>                         (nr_range - 1 - i) * sizeof (struct map_range));
>> -               mr[i].start = old_start;
>> +               mr[i--].start = old_start;
>>                nr_range--;
>>        }
>>
>
>this patch seems not right.
>Ingo, please don't apply it.
>
>original code:
>        /* try to merge same page size and continuous */
>        for (i = 0; nr_range > 1 && i < nr_range - 1; i++) {
>                unsigned long old_start;
>                if (mr[i].end != mr[i+1].start ||
>                    mr[i].page_size_mask != mr[i+1].page_size_mask)
>                        continue;
>                /* move it */
>                old_start = mr[i].start;
>                memmove(&mr[i], &mr[i+1],
>                         (nr_range - 1 - i) * sizeof (struct map_range));
>                mr[i].start = old_start;
>                nr_range--;
>        }
>
>so it save old_start and first, and move entries forward (so old one
>is overwriten), and put back old_start ...

Old and new code are not different in any way in this respect - both
overwrite the old entry at index i with the entry at index i+1 and then
set the start of the i-th entry back to what it was before the overwrite,
effectively combining them. The patch just makes sure that the index
isn't being updated at the same time as nr_range (because if you update
both you effectively skip one).

The issue is apparently pretty benign to native code, but surfaces as a
boot time crash in our forward ported Xen tree (where the page table
setup overall works differently than in native). Since the underlying
issue was present in native (and since I assume if there is an attempt
to merge subsequent regions, then it should work right), I nevertheless
submitted the patch for native inclusion.

Jan

Re: [PATCH] x86-64: fix combining of regions in init_memory_mapping()

[Yinghai Lu]

On Mon, Sep 15, 2008 at 1:17 AM, Jan Beulich <jbeulich@novell.com> wrote:
>>>> "Yinghai Lu" <yhlu.kernel@gmail.com> 14.09.08 20:20 >>>
>>On Fri, Sep 12, 2008 at 7:43 AM, Jan Beulich <jbeulich@novell.com> wrote:
>>> When nr_range gets decremented, the same slot must be considered for
>>> coalescing with its new successor again.
>>>
>>> Signed-off-by: Jan Beulich <jbeulich@novell.com>
>>>
>>> ---
>>>  arch/x86/mm/init_64.c |    2 +-
>>>  1 file changed, 1 insertion(+), 1 deletion(-)
>>>
>>> --- linux-2.6.27-rc6/arch/x86/mm/init_64.c      2008-08-29 10:53:00.000000000 +0200
>>> +++ 2.6.27-rc6-x86_64-mr-coalesce/arch/x86/mm/init_64.c 2008-09-12 11:58:45.000000000 +0200
>>> @@ -636,7 +636,7 @@ unsigned long __init_refok init_memory_m
>>>                old_start = mr[i].start;
>>>                memmove(&mr[i], &mr[i+1],
>>>                         (nr_range - 1 - i) * sizeof (struct map_range));
>>> -               mr[i].start = old_start;
>>> +               mr[i--].start = old_start;
>>>                nr_range--;
>>>        }
>>>
>>
>>this patch seems not right.
>>Ingo, please don't apply it.
>>
>>original code:
>>        /* try to merge same page size and continuous */
>>        for (i = 0; nr_range > 1 && i < nr_range - 1; i++) {
>>                unsigned long old_start;
>>                if (mr[i].end != mr[i+1].start ||
>>                    mr[i].page_size_mask != mr[i+1].page_size_mask)
>>                        continue;
>>                /* move it */
>>                old_start = mr[i].start;
>>                memmove(&mr[i], &mr[i+1],
>>                         (nr_range - 1 - i) * sizeof (struct map_range));
>>                mr[i].start = old_start;
>>                nr_range--;
>>        }
>>
>>so it save old_start and first, and move entries forward (so old one
>>is overwriten), and put back old_start ...
>
> Old and new code are not different in any way in this respect - both
> overwrite the old entry at index i with the entry at index i+1 and then
> set the start of the i-th entry back to what it was before the overwrite,
> effectively combining them. The patch just makes sure that the index
> isn't being updated at the same time as nr_range (because if you update
> both you effectively skip one).
>
> The issue is apparently pretty benign to native code, but surfaces as a
> boot time crash in our forward ported Xen tree (where the page table
> setup overall works differently than in native). Since the underlying
> issue was present in native (and since I assume if there is an attempt
> to merge subsequent regions, then it should work right), I nevertheless
> submitted the patch for native inclusion.

yes. your patch fixed the skip...

Acked-by: Yinghai Lu <yhlu.kernel@gmail.com>

YH

Re: [PATCH] x86-64: fix combining of regions in init_memory_mapping()

[Ingo Molnar]

* Yinghai Lu <yhlu.kernel@gmail.com> wrote:

> > The issue is apparently pretty benign to native code, but surfaces 
> > as a boot time crash in our forward ported Xen tree (where the page 
> > table setup overall works differently than in native). Since the 
> > underlying issue was present in native (and since I assume if there 
> > is an attempt to merge subsequent regions, then it should work 
> > right), I nevertheless submitted the patch for native inclusion.
> 
> yes. your patch fixed the skip...
> 
> Acked-by: Yinghai Lu <yhlu.kernel@gmail.com>

applied to tip/x86/core, thanks!

	Ingo